What are molecules made from? They are made from atoms, which are themselves made from nuclei and electrons. These building blocks carry an electrical charge: nuclei are positively charged, and electrons are negatively charged. The nuclei themselves are made up of (positively charged) protons and (neutral) neutrons. This is all summarised on the following picture:
Different types of atom have different numbers of protons, neutrons, and electrons. For example, carbon atoms have 6 protons, 6 neutrons, and 6 electrons.
Charged species interact with each other: like charges (+ and + or - and -) repel each other, opposite charges (- and +) attract each other. This well-known principle from physics is summarised by Coulomb's law:
![\[ F = \frac{1}{4\pi\epsilon_0} \frac{q_1q_2}{r^2} \]](basic_princ_1.gif)
(Here, F is the force between the two charges; ε0 is a constant (not important here), q1 and q2 are the values of the charges involved, and r is the distance between them.)
This force between charged species is central to all of chemistry, and in particular to all the types of bonding we will discuss.
First, it explains how atoms hold together: the negatively charged electrons are attracted to the positively charged nucleus more than they are repelled by the other electrons.
There is a fine balance between the attractive force holding the electrons close to the nucleus, and the repulsive force which tends to keep electrons away from each other. The result of this competition between attractive and repulsive charge-charge interactions is what explains the detailed structure of atoms. The electrons in atoms tend to form into concentric shells. For the hydrogen atom, with just one electron and one proton (Z = 1), the electron sits in the first shell, as shown here:
The nucleus is shown in purple. Also shown is the structure of the Helium atom, with two protons, two neutrons (all shown together as the purple nucleus), and 2 electrons (i.e., Z = 2). Both electrons sit in the first shell.
For elements with more electrons, there is no more room in the first shell, and so a second shell is occupied. This is shown below for carbon (Z = 6) and oxygen (Z = 8).
Above 10 electrons, the second shell contains eight electrons and is full. For the elements beyond (starting with sodium, Z = 11), the last electrons therefore sit in the third shell, as shown here for sodium and chlorine (Z = 17):
This structural description leads naturally to an important property of atoms, the octet rule: atoms have a strong tendency to lose, gain, or share electrons if this leads to them having a complete shell of electrons around them. In other words, atoms prefer to have a total of 2, 10, or 18 electrons around them.
In the next pages, we will discuss how interaction between charges on atoms leads to bonding.
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